Electrochemical sensor

ABSTRACT

A electrochemical sensor comprising a mounting having screen printed array of electrodes located thereon, the array comprising a reference electrode, a counter electrode and a plurality of working electrodes, wherein the working electrodes are each overlaid with an insulating layer of insulating material, the insulating layer having an array of apertures, exposing a respective array of working regions of the working electrodes, and a method of making same by applying screen printing technic, and a method of using same for chlorine determination.

This invention relates to an electrochemical sensor for detection of an analyte, particularly but not exclusively for detection of chlorine. The invention also relates to a method of detection of chlorine or other analyte and to a method of manufacture of a sensor for use in performance of the method. In alternative embodiments of the invention, the sensor may detect ammonia.

Chlorine in aqueous solution is a powerful oxidising agent and is used widely for the disinfection of drinking water, recreational water and in the treatment of industrial water. If properly applied, chlorination also provides other benefits such as the removal of colour, taste and odour control, and the prevention of biological growth.

It is essential to control the level of chlorine in water so that the most effective concentration can be maintained. The presence of excess chlorine is detrimental to human and aquatic life and can produce chlorinous tastes and odours in drinking water and unpleasant bathing conditions in swimming pools.

Chlorination may occur by the addition of gaseous chlorine to water or by the addition of sodium hypochlorite. Chlorine rapidly dissolves in water to form hypochlorous acid and hydrochloric acid:

Cl₂+H₂0

HCl+HOCl  (1)

Sodium hypochlorite also reacts to form hypochlorous acid:

NaOCl+H₂0

NaOH+HOCl  (2)

The hypochlorous acid partially dissociates to give the hypochlorite ion:

HOCl

H⁺+OCl⁻  (3)

The three forms of chlorine involved in reactions 1-3, molecular chlorine (Cl₂), hypochlorous acid (HOCl) and hypochlorite ion (OCl⁻) may exist together. Their relative proportions depend upon the pH and temperature. Chlorine in any of these forms is knows ad the “free available residual chlorine”.

When free chlorine is added to water, containing ammonia, the hypochlorous acid reacts with the ammonium ion, and depending on the pH, temperature and initial chlorine to ammonia ratio, leads to the formation of monochloramine (NH₂Cl), dichloramine (NHCl₂) and nitrogen trichloride (NCl₃). Chlorine in any of these forms is knows as the “combined available residual chlorine”.

Chlorine in the form of HOCl exhibits the greatest bactericidal activity and combined chlorine generally is a much weaker disinfectant. It is therefore important to be able to measure and distinguish between the free and combined forms of chlorine.

There are several laboratory methods for the determination of free and combined chlorine, for example iodometric and amperometric titrations, redox titrations using N,N-diethyl-p-phenylenediamine (DPD) as an indicator, and calorimetric methods using DPD or syringaldazine. However, the chlorine content of water dissipates very quickly and it is important therefore that testing should be done with a minimum of delay and preferably at the point of sampling. A number of portable field test kits have been developed; the most common and widely used is the DPD calorimetric procedure. The reagents are supplied in the form of tablets, powders or liquids and the colour formed by the reaction of free chlorine with DPD is measured spectrophotometrically. Total chlorine can be determined by the further addition of potassium iodide, which induces combined chlorine to react. The combined fraction can then be calculated by the difference between the total and the free chlorine in the sample.

The disadvantages of such a field test include the necessity for the sample to be colour and turbidity free, the use of reagents can introduce errors due to poor mixing, free and combined chlorine must be determined separately and the method is susceptible to a number of interferences.

Free chlorine and total chlorine can be measured electrochemically by applying a voltage to an electrode and measuring the current. Free chlorine can be measured directly; the reaction at the electrode can be represented by:

HOCl+2e ⁻→Cl⁻+OH⁻  (4)

Total chlorine (free and combined) can be measured by the addition of potassium iodide, Reactions (5) and (6) show the reaction of potassium iodide with free chlorine and combined chlorine:

2I⁻+Cl₂→I₂+2Cl⁻  (5)

2H⁺+NH₂Cl+2I⁻→NH₄ ⁺Cl⁻+I₂  (6)

The iodine is then reduced to iodide at the electrode.

I₂+2e ⁻2I⁻  (7)

The combined chlorine can then be calculated by the difference in free and total chlorine. Such a system requires reagents to buffer the pH of the test sample, to provide electrolyte and to provide potassium iodide for the measurement of total chlorine. It also requires frequent calibration to correct for drift in the response of the electrode.

WO-A-91/08474 discloses a microelectrode manufactured by use of a photoablation technique to create apertures in a layer of electrically insulating material and allow electrically conducting material exposed through the apertures to create a microelectrode.

According to a first aspect of the present invention, an electrochemical sensor comprises a mounting having a screen printed array of electrodes located thereon, the array comprising a reference electrode, a counter electrode and a plurality of working electrodes, wherein the working electrodes are each overlaid with an insulating layer of insulating material, the insulating layer having an array of apertures exposing a respective array of working regions of the working electrodes.

The sensor is preferably adapted for detection of chlorine, more preferably for the simultaneous measurement of free and total chlorine without requiring use of any additional reagents or for calibration by a user.

Disposable mountings may be provided.

A preferred sensor may comprise a laminar sheet or strip of insulating polymeric material upon which successive layers are applied by screen printing.

In preferred embodiments of the invention, the apertures may have a dimension, or preferably a diameter of about 50-400 μm, preferably about 100-200 μm. Circular apertures are preferred.

In preferred embodiments, the array may comprise 10-500 apertures, preferably 50-200 apertures, more preferably 80-120, most preferably about 95 apertures.

The working electrodes are preferably composed of carbon and may be made by screen printing.

A sensor in accordance with the present invention confers several advantages. A small overall working electrode area may be used so that the total current passed by the electrode is small, making it possible to operate in a solution without addition of a supporting electrolyte. An enhanced rate of mass transport can be achieved using an array of smaller electrodes. Furthermore, a reduction in the double layer capacitance is obtained, leading to an improved level of detection.

A reagent layer may be provided overlying the apertures in the insulating layer. The reagent layer may comprise a porous layer impregnated with one or more reagents adapted to form an electrochemically detectable species when contacted with an analyte. The reagent may comprise a redox indicator that reacts with free chlorine. Alternatively or in addition, the reagent may comprise potassium iodide to react with total chlorine.

Preferred sensors in accordance with this invention do not require addition of reagents to the test solution, for example to determine free and/or total chlorine. Free and total chlorine can be measured simultaneously.

Alternative sensors may be adapted to measurement of ammonia by electro-deposition of a conducting polymer e.g. polypyrrole, polyaniline or polythiophene on the carbon working electrode or the printing of a conducting polymer containing ink as the working electrode.

The reagent may be deposited in the reagent layer by printing or microdosing.

The invention is further described by means of example, but not in any limitative sense, with reference to the accompanying drawings, of which:

FIG. 1( a) is a plan view of a screen-printed sensor in accordance with the present invention;

FIG. 1( b) is a cross-section of a working electrode of a sensor shown in FIG. 1; and

FIG. 2 is a plot of current versus time for 0 ppm and 1 ppm chlorine using a free chlorine electrode.

The electrode shown in FIGS. 1( a) and (b) comprises a support strip composed of insulated polymeric material (1) and a silver or other metallic conductive track (2) deposited on the strip (1). A) The polymer layer may comprise polyester, polycarbonate or polyvinyl chloride. Each layer is deposited on the plastic substrate by screen printing (also known as silk screen printing or thick film printing). Other suitable processes for deposition of the layers include lithography, vapour deposition, spray coating and vacuum deposition. The first layer consists of four parallel conductive tracks (2) deposited using a highly conductive printable ink formulation. A silver based ink or a silver/silver chloride based ink may be used. One of the four tracks acts as a reference electrode (6). Two working electrodes (7, 8) and a counter electrode (5) are formed by depositing parallel rectangles of another conducting ink over the silver tracks. The conducting ink in this case may be carbon but gold or other metal may be employed. The preferred shape of the electrodes in rectangular but any convenient shape, for example square or circular may be used. The final printing stage involves the deposition of an electrically insulating material over the carbon electrodes. The insulating layer has a number of apertures, for example 95 or more, that exposed the underlying carbon electrode to provide a corresponding array of 95 or more carbon electrodes. The apertures were all of the same size and had a diameter of 200 μM. Electrodes having smaller aperture sizes in the range 50 to 400 μM may be used. The apertures are preferable round but may have any convenient shape.

In a preferred embodiment adapted to simultaneously measure free and total chlorine, liquid reagents are deposited into the reagent layer of each working electrode using a liquid dispensing system. A Biodot AD32000 platform and Biojet Plus 3000 dispensing system may be employed. The total chlorine measuring electrode may have a reagent layer impregnated with potassium iodide (4% w/v) and potassium phthalate (0.1M, pH4) and hydroxyl-ethyl cellulose (0.5% w.v.).

Although free chlorine can be measured directly at a carbon electrode, the kinetics of the reaction are poor and a large over-voltage is required. This leads to a high background signal, poor sensitivity and an increased chance of interference from other reduceable species which may be present in the analyte. To overcome this problem, a redox indicator, for example, DPD or tetramethylbenzidine (TMB), that reacts to the free chlorine may be employed. TMB is preferred because of the stability in solution. A solution used to impregnate a free chlorine electrode contains TMB (1 mg/ml), sodium phosphate buffer (0.2M, pH7) and polyvinylpyrrolidone (1%).

In use of the electrode a voltage of −0.2V versus the printed reference electrode was applied and the current from each working electrode was monitored for 60 seconds by chronoamperometry. The results are shown in FIG. 2. The analytical response was calculated by integrating the current over part or all of the test period. A fuseable link may be incorporated into each sensor so that the sensor can only be used once.4 

1. An electrochemical sensor comprising a mounting having a screen printed array of electrodes located thereon, the array comprising a reference electrode, a counter electrode and a plurality of working electrodes, wherein the working electrodes are each overlaid with an insulating layer of insulating material, the insulating layer having an array of apertures exposing a respective array of working regions of the working electrodes, wherein a reagent layer overlies said apertures within the insulating layer.
 2. A sensor as claimed in claim 1 adapted for use in detection and/or assay of an analyte.
 3. A sensor as claimed in claim 1, in which the analyte is at least one form of chlorine.
 4. A sensor as claimed in claim 1 in which the analyte is at least one form of ammonia.
 5. A sensor as claimed in claim 3 adapted for use in simultaneous measurement of free and total chlorine and/or free and combined chlorine.
 6. A sensor as claimed in claim 4, wherein a conductive polymer is present on a working electrode or a conductive polymer layer has been printed onto the sensor containing conductive ink as a working electrode.
 7. A sensor as claimed in claim 1, including working electrodes of carbon.
 8. A sensor as claimed in claim 7, in which the said working electrodes of carbon have been screen printed onto the sensor.
 9. A sensor as claimed in claim 1, including four generally parallel tracks comprising a reference electrode, two working electrodes and a counter electrode.
 10. A sensor as claimed in claim 1, which is disposable, for example, by the inclusion therein of a fuseable link.
 11. A sensor as claimed in claim 1, comprising a laminar sheet or strip of insulating polymeric material upon which successive layers have been applied by screen printing.
 12. A sensor as claimed in claim 1, wherein the apertures in said array have a dimension of 50 to 400 μM.
 13. A sensor as claimed in claim 12, wherein the said dimension is 100 to 200 μM.
 14. A sensor as claimed in claim 1, wherein the apertures are circular.
 15. A sensor as claimed in claim 1, wherein the array comprises 100 to 500 apertures.
 16. A sensor as claimed in claim 15, wherein the array comprises 50 to 200 apertures.
 17. A sensor as claimed in claim 10, wherein the array comprises 80 to 120 apertures.
 18. A sensor as claimed in claim 11, wherein the array comprises 95 apertures.
 19. A sensor as claimed in claim 1, wherein a plurality of tracks of highly conductive printable ink have been screen printed onto the mounting, each in contact with an electrical contact of the sensor.
 20. A sensor as claimed in claim 19 in which the ink is of silver or is silver chloride based,
 21. A sensor as claimed in claim 19, wherein working electrodes of the sensor deposited on said tracks are of carbon or gold.
 22. A sensor as claimed in claim 1, in which the reagent layer comprises a porous layer impregnated with one or more reagents.
 23. A sensor as claimed in claim 1, adapted for use in the assay of total chlorine present in a test solution.
 24. A sensor as claimed in claim 1, in which said reagent layer is impregnated with an iodide such as potassium iodide, and with or without a buffer such as potassium phthalate.
 25. A sensor as claimed in claim 1, in which the reagent layer includes a redox indicator able to react with chlorine in its free form.
 26. A sensor as claimed in claim 25, in which the said redox indicator comprises DPD or TMB optionally in the presence of a buffer, for example, phosphate buffer.
 27. A sensor as claimed in claim 26, in which the reagent layer further comprises polyvinylpyrrolidone and/or hydroxyethylcellulose.
 28. A sensor as claimed in claim 1, in which the reagent layer has been deposited by screen printing or microdosing,
 29. A method of making a sensor as claimed in claim 1, comprising providing a mounting in the form of insulating polymeric material such as plastics material, depositing a silver or other metallic conductive track thereon and wherein each electrode layer as defined in claim 1 is deposited upon the sensor by screen printing.
 30. A method as claimed in claim 29, in which a deposition stage involves deposition of the insulating layer over the working electrodes such that said layer is provided with said array of apertures exposing working regions of said electrodes.
 31. A method as claimed in claim 30, in which said deposition is effected by screen printing of the said insulating layer with an array of microapertures.
 32. A method as claimed in claim 29, wherein additionally one or more liquid reagents are deposited upon the sensor within a reagent layer of a working electrode.
 33. A method as claimed in claim 32, in which the reagent(s) is (are) absorbed into said layer and allowed to dry before use.
 34. A method of detecting an analyte in a test solution involving use of a sensor as claimed in claim
 1. 35. A method as claimed in claim 34, including assessing the analyte,
 36. A method as claimed in claim 34, wherein the analyte comprises at least one form or chlorine and/or ammonia,
 37. A method as claimed in claim 34, in which the sensor includes a reagent layer.
 38. A method as claimed in claim 37, carried out on a test solution in the absence of other assay reagents in the test solution.
 39. A method as claimed in claim 37, carried out in the absence of additional or supplementary calibration by the user of the method.
 40. A method as claimed in claim 34, for the detection of free and total chlorine species in an aqueous test solution or free and combined chlorine species therein.
 41. A method as claimed in claim 34, for the detection of ammonium species in an aqueous test solution. 